Finely chop 1/2 cup onion and next 2 ingredients in a blender or food processor. Add olive oil, mango and lime juice and pulse until combined. Pour 1/2 cup mint sauce into a large resealable plastic bag with Knorr® Beef flavor Bouillon and steak. Close bag and marinade for 30 minutes in refrigerator.
Remove steak from marinade, discarding marinade. Grill or broil steak until desired doneness, turning once.
Arrange greens, tomatoes and remaining onions on a serving platter, then top with thinly sliced steak. Drizzle with remaining mint sauce.